1. Field of the Invention
The invention relates to a power generation system which utilizes naturally occurring low grade heat energy at or near the earth's surface to produce mechanical or electrical power.
2. Prior Art
Systems for generating power convert the thermal energy difference between a heat source and a heat sink to useful power by driving a generator or other power output while transferring heat energy from the source to the sink. Such systems are most efficient where the difference in the temperature between the source and sink is the greatest. Geothermal power generation systems are known which rely on heat from high temperature sources, located in an area of volcanic activity and/or far below the earth's surface, generally at depths of from 100 to 30,000 feet. The heat is extracted from maqma or superheated rock, and carried to the surface by water, brine, etc. The heat is then extracted at the surface, and various uses can be made of the heat, including operating a turbine or other device coupled to an electric generator. Whereas the surface temperature is always lower than such high temperature sources, the heat extraction technique can be used to generate power.
A power generation system of this type may use a circulating coolant which is changed between a liquid phase and a gas phase in each pass around the circulation loop, the changes being a result of the temperatures encountered at the source and at the sink. For example, pressurized liquid coolant is heated and phase changed into a gas on the hotter side of a circulation loop, and after driving a turbine-generator or the like at which the heated coolant is allowed to expand and cool, the now-gaseous coolant is condensed and depressurized to the liquid phase on the cooler side of the circulation loop, proceeding again around the loop to the hotter side. The coolant is circulated continuously around the loop, generally using the temperature difference between the hotter and cooler sides as the power source for driving the turbine.
Apart from direct association with volcanic activity, the underground temperature of the earth near the surface, where insulated from day to day surface temperature variations, is a relatively stable temperature in the mid 50.degree. Fahrenheit range. However, at any particular time of the year the temperature at the surface, specifically the surface air temperature, may be higher or lower than the temperature of the earth beneath the surface. The earth's underground temperature also increases roughly 88.degree. Fahrenheit per mile of depth.
It is known to use the temperature difference between a heat source or sink at a temperature nearly equal to the surface air temperature to move heat energy into or out of a building or other heating/cooling load, by use of a similar circulating coolant system known as a heat pump. The coolant is heated (or cooled) at a heat exchanger located outside of a building and the heat is extracted (or the coolant extracts heat) at a heat exchanger disposed in the building, one or both heat exchangers normally being associated with fans for moving air over the heat exchanging surfaces. Such systems do not produce mechanical or electrical energy from the temperature difference. The systems simply extract heat or sink heat between the building and the outside heat exchanger, using electrically powered fans and pumps.
Known systems for tapping the ever abundant heat source of the earth below the surface, for the purpose of generating power, typically convert water into steam for driving a turbine or operating a refrigeration plant. For example, see U.S. Pat. Nos. 4,091,623; 4,142,108; 4,189,923; 4,255,933; and 4,388,807. The systems require passages leading deep into the earth. Drilling expenses, passage obstruction problems, shifting of the earth associated with volcanic activity, and other expenses or technological problems generally render these ostensibly good ideas economically unrealistic and infeasible.
Power generation systems have also been developed to utilize temperature differentials due to the cooling of ocean water at depth, or as provided due to prevailing currents. Generally, ocean water near the surface, which is warmed by the sun, provides the heat source, and colder deeper ocean water provides a significantly cooler temperature differential, enabling the generation of mechanical and electrical power. See, for example, U.S. Pat. Nos. 4,087,975; 4,189,924; and 4,302,682. Oceanic thermal difference energy conversion systems are theoretically attractive for generating power, however their application is obviously limited to ocean areas and the cooler-side heat exchanger must be very deep to obtain a substantial temperature difference compared to the surface temperature. As with the deep well geothermal systems, from a practical standpoint these proposed power generation systems are quite large, as considered necessary to be economically feasible in many applications. Also, major potential problems remain, including weather problems (e.g., hurricanes and typhoons), tides, shipping traffic, barnacles, corrosion due to long-term exposure to sea water, etc.
In U.S. Pat. No. 4,290,266, a coolant or refrigerant line is placed sufficiently deep for geothermal heat to convert gravity drained liquid refrigerant into a gas under high pressure for use in driving a turbine. The concept of using a phase changing coolant or refrigerant other than water is the same as that of changing water into steam and back for driving a turbine in a geothermal system. However, the refrigerant is normally chosen such that it has a boiling point suitable for the temperature levels of the system design and thus is readily changed in phase by the temperatures encountered on the hotter and cooler sides of the loop. On the other hand, the refrigerant concept has significant operational problems when one attempts to apply it to geothermal power generation due to the requirement for substantial vertical conduit lengths between the surface and the subsurface heat exchangers. For example, there is no ready means to force the refrigerant dependably under power around the circulation loop, due to the phase differences between the heavier and less compressible liquid and the lighter and more-compressible gas phases. Once gravity fed liquid refrigerant is phase changed into a gas, it is difficult to force it further downward, because the low density gas tends to rise in the higher density liquid refrigerant. In short, it is difficult to move the refrigerant in a loop to a sufficiently hot subterranean depth, prior to phase change, for naturally producing the pressures needed for significant power generation. Once the liquid refrigerant changes phase from a liquid to a gas, the effects of gravity flow are substantially diminished. Inasmuch as the most remote point in the circulation loop may be far below the surface, such gravity flow refrigerant systems ultimately suffer from poor system equilibrium and periods of in operation. Other problems include inability of the system to operate in a reverse direction, inability to recover geothermal heat on the heat extraction side after extended operating periods, and typically, high costs and associated problems encountered with deep wells.
Another approach for a refrigerant type power generation system is disclosed in U.S. Pat. No. 3,995,429 (Peters). This patent describes the production of a pressurized vapor via selective utilization of temperature differentials in two of three or more heat sources or sinks, each of which varies in temperature over time. A fluid pump is powered by an electric motor for moving liquid refrigerant around a loop including selected ones of the sources and sinks. Controls and valves are provided for switching between heat and heat sink sources having the most efficient (highest) naturally occurring temperature differential. The disclosure of the patent teaches heat sources including a solar energy absorber, a radiator placed in the earth or in water, and an atmospheric heat exchanger. One obvious problem with this system, as noted in the patent, is that under certain conditions there is no sufficient temperature difference between any two of the heat, or heat sink, sources, whereupon all action stops. This results in a lack of continuous and dependable power. Moreover, refrigerant equilibrium problems and imbalances in refrigerant quantities occur and must be accurately and consistently controlled to effect appropriate refrigerant phase changes under varying load and temperature differential conditions and to effectively generate power. For example, Peters neglects to provide a means to overcome the negative effects which will operationally be realized when the vaporized refrigerant encounters pressure resistance from the turbine and exerts a back pressure against and/or into the liquid refrigerant exiting the liquid refrigerant pump and/or source. Such back pressure can severely hamper system operational efficiency, ultimately placing as much of a power drain on the circulating pump as the turbine is able to produce from the coolant. This back pressure can result in system equilibrium loss and shut down. Further, the Peters invention neglects to provide for a coolant accumulator/dispenser system which will automatically sense conditions and adjust the amount of refrigerant contained in the circulation loop at any given time to maintain optimum conditions for the particular heat source/heat sink system being utilized for power generation. Such a refrigerant supply control is critical for optimum system operation under varying temperature conditions. For example, when operating under relatively colder temperature conditions, a larger quantity of refrigerant is required to achieve optimum performance than when operating under relatively hotter temperature conditions. If refrigerant quantities in the system are not controlled and reduced when operating between the warm sun and warm air and/or warm water and/or warm earth in the summer, as opposed to colder identical heat and heat sink sources in the winter, pressures may become so high or so low as to result in pump and/or generator burn out or malfunction.
The present invention overcomes these problems by providing a geothermal power system and method which utilizes a low grade, naturally occurring heat source found at or near the surface of the earth. The invention provides a heat exchanger having two or more compartmentalized heat exchanger cells in contact with the naturally occurring heat source for vaporizing a liquid refrigerant. The heat exchanger cells are spaced apart, and switchable valves are provided for selectively controlling refrigerant flow through the individual cells so that the heat source in the vicinity of each cell can alternately be given time to naturally recover heat after being drawn down by refrigerant vaporization. An accumulator/dispenser is provided with means to monitor and control refrigerant quantities in the system in order to maintain optimum system temperatures and pressures and to prevent dangerous system overpressures or underpressures. A supplemental design utilizing two separate refrigerant loops may also be used, with the first loop utilizing a compressor for circulating the refrigerant, and for pressurizing and heating the vaporized refrigerant returning from the underground heat source, to a suitable temperature for ensuring a phase change in the secondary refrigerant loop for operating a turbine, reciprocating engine or other power extraction device which expands the pressurized gas and assists in converting the pressurized refrigerant back into the liquid phase. The turbine or other engine may be coupled to a generator for producing electricity. Alternatively, it can be used to provide mechanical power. The mechanical or electrical energy can be stored via compression of gas or hydraulic fluids, electrolysis, batteries, etc., for later use. Expansion valves and condensing cells are provided as necessary to maintain operational temperature/pressure differentials in the system. For the system with two separate refrigerant loops wherein the refrigerant in the first loop is continuously circulated by the compressor, the system, unlike Peters, is operational with extremely modest temperature differentials between the heat source and the heat sink.
The invention is particularly applicable to a ground source heat exchanger having temperature exchange coils which are buried in an array near the earth's surface, especially just below the frost line or heat line. For example, a sinuous pattern of copper or other thermally conductive tubing can be mounted along the walls of a simple backhoe trench to provide the subsurface heat exchanger. Similarly, a bored cylindrical hole can be lined with a helical pattern of heat exchange coils, rested along the sides of the hole before backfilling. The subsurface heat exchanger can be operated in conjunction with heat exchangers for the opposite side of the loop, including water based and ambient air heat exchangers, thus providing a versatile system for extracting power from naturally occurring temperature differences. The apparatus can be provided simply as a power generation device, or alternatively can be operated in a power generation mode only when not in use as a building HVAC heat pump system.